A known diagnostic technique used in tomography for locating tumors involves injecting into a patient's bloodstream a radioactive isotope which targets the tumor, so that the location of the tumor can be derived by detecting the location of the radioactive isotope. Typically, the radioactive isotope emits γ-rays which are dispersed from the tumor site. In order to achieve the desired detection so as to determine the precise location of the tumor, it is necessary to image the patient's body in such a manner as to detect only those γ-rays which are emitted normally from the body and to ignore those γ-rays which are dispersed in other directions.
Different types of computer tomography are known in which such a radiation imaging system may be embodied. In Single Photon Emission Computed Tomography (SPECT) more than one detector is rotated around the subject and the radioisotope's distribution (tomographic image) is reconstructed based on an obtained count values of the γ-rays.
In contrast to SPECT where a radioisotope in the body emits γ-rays produced by a single photon, in Positron Emission Tomography (PET) a patient is administered a radioisotope that emits positrons (i.e. positively charged electrons). When the positrons meet electrons within the body, the positrons and electrons mutually annihilate and produce two γ-rays that propagate away from each other at an angle of 180° and are detected by respective detector segments in the PET scanner. The scanner's readout electronics record the detected γ-rays and map an image of the area where the radioisotope is located. Here also two simultaneous detections are indicative of a positron emission from the tumor site.
Ideally, if during every scan of the composite image sensor, each pixel is read sequentially only one at a time, then the current scan in each segment can be terminated when an “active” pixel is detected assuming that only pixel in each segment can be active. However, two factors militate against this ideal approach. First, it is impractical to read each pixel in such a manner because of the time overhead involved in addressing each pixel separately and downloading the pixel data along a dedicated channel for further processing. Secondly, this ideal approach assumes that each pixel corresponds to a strike by single photon. However, in practice, the energy of an impinging γ-ray may not be totally absorbed by a single pixel but may be shared by more than one pixel. This will occur, for example, when the γ-ray strikes a pixel off-center so that its energy is shared by a central pixel (usually absorbing most of the energy) and one or more neighboring pixels, which together absorb the residual energy. It can also occur owing to Compton scattering, which may occur in any high-energy particle detector as explained above but forms the principle of operation in a Compton camera.
For the purpose of the present discussion, a Compton camera may be regarded as just another type of 2-dimensional image sensor having a plurality of addressable pixels, one of which emits a signal when stimulated by a γ-ray. Specifically, each pixel is a diode which generates a charge signal when hit by a γ-ray. A γ-ray emitted by the radioisotope will be detected only if it creates a Compton effect by creating a charge signal thereby giving up some of its energy. In practice, it is usual to employ a composite sensor having several spaced-apart sensor layers each containing at least one sensor module so as to increase the probability that an incident γ-ray will produce a Compton effect in at least one of the layers. The multi-layer sensor module constitutes a first detector of the Compton camera. Having thus produced a Compton effect, the γ-ray then emerges from the first detector. However, in order to calculate the angle of the incident γ-ray, the emergent γ-ray is directed to a second detector in which it is absorbed completely, thereby giving up all of its residual energy. Such a detector is described in EP 893 705 published on Jan. 27, 1999 entitled “Multi-Channel Readout circuit for Particle Detector” and assigned to the present applicant.
Photons produced by Compton scattering are formed substantially simultaneously. The scattering process by which this typically occurs is that the photon interacts with an electron (primary hit), but not all of its energy is deposited. The photon changes direction, and may deposit the remainder of the energy in a different pixel with a secondary interaction with an electron. Thus, if two active pixels are detected simultaneously and their combined energy is conducive with their being derived by Compton scattering from an incident γ-ray having an energy of 511 keV, then the position of each energy emission allows a collision line between the locations of the first and second collisions to be determined, and their respective energies allow determination of the angle of Compton scattering.
It is thus desirable to read two or more active pixels in a pixel array without the need to read all the pixel data so as to reduce the time required to perform computer tomography and hence the time for which a patient is exposed to radiation.
One known approach to doing this is to use so-called sparse readout, such as described in U.S. Pat. No. 5,847,396 (Lingren et al.) assigned to Digirad Corporation, which discloses a high-energy photon imaging system comprising an imaging head that includes a detector having a plurality of detection modules. Each detection module comprises a plurality of detection elements fixed to a circuit carrier. The circuit carrier includes channels for conditioning and processing the signals generated by corresponding detection elements. Each channel stores the amplitudes of the detection element electrical pulses exceeding a predetermined threshold. The detection modules employ a fall-through circuit, which avoids the need for sequential readout and automatically finds only those detection elements whose stored pulse amplitude exceeds the threshold. The fall-through circuit searches for the next detection element and associated channel having a valid event, meaning that the detection element exhibits a pulse magnitude that exceeds a certain threshold. Use of sparse readout in a PET camera is also described in U.S. patent application Ser. No. 09/827,439 filed Apr. 5, 2001 in the name of the present assignee and entitled “Improved Readout circuit for a Charge Detector”.
Another approach that may be used independently of sparse readout, or in addition thereto, is described in U.S. Pat. No. 5,825,033 (Barrett et al.) published Oct. 20, 1998 and entitled “Signal processing method for gamma-ray semiconductor sensor”, which relates to the need to read sub-threshold data of neighboring pixels. In order to do so, all pixels must be read out in order to define a “central pixel” and a “neighborhood” of related neighboring pixels whose data must be also be read since there exists an a priori likelihood that charge is shared between the central pixel and one or more of the neighboring pixels. Specifically, it is to be noted that the voltage signal for each pixel must be compared to a corresponding predetermined threshold in order to identify all pixels having an above-threshold voltage signal. Thereafter, clusters of adjacent pixels are identified having above-threshold voltage signals and a cumulative voltage signal associated with the cluster is calculated. Thus, U.S. Pat. No. 5,825,033 is not related to a sparse readout system that attempts to reduce the number of pixels whose voltage signals must be read, but rather reads all pixels in the pixel array.
U.S. Pat. No. 5,107,122 (Barkan et al.) published Apr. 21, 1992 and entitled “Sparse readout method and apparatus for a pixel array” does relate to sparse readout of a pixel array where outputs are obtained only from selected pixels. These pixels are determined by those pixels that have received actuating inputs, and may consist of the hit pixels and their immediate neighbors. The system obtains outputs from the selected pixels only for the times that correspond to the occurrence of an event of interest. By such means, the amount of data to be processed is substantially reduced. A content addressable memory is used to store the times when photons strike pixels, whose locations in the pixel array are stored in a random access memory. Active pixels are read sparsely column-by-column and on reaching an active pixel, the pixel in each row in the active column for which a hit occurred since the time of the previous event of interest is accessed and compared with the row addresses of the pixels hit at the time of the current event of interest. By such means associated pixels may be read being those that are hit at the same time as the current event of interest. However, these pixels must be in the same pixel array and thus the readout method is suitable for charge sharing but may not be suitable for Compton scattering where charge can be scattered between spatially separated pixel arrays.
It is thus to be noted that neighboring pixels are determined on the fly according to the time stamp associated with each event. This allows associated pixels to be determined on the basis of simultaneity of photon impingement. Although sparse readout is used to identify active columns and thus avoids the need to read each pixel sequentially, there is no attempt to define a priori for each pixel a subset of neighboring pixels, being those most likely to be associated with an active pixel.
Another problem in all readout circuits relates to dead time in the system. When reading out one pixel in a sampled system, all pixels that are sampled but not read are also “dead”. As such, if during the act of sampling these pixels, a different photon strikes one of the sampled pixels, this event will be lost. It is impossible to eliminate dead time altogether but by sampling only a subset of the pixels, the other pixels should in principle be in an active “reading-mode” and so the effects of dead time are reduced. This is true also in U.S. Pat. No. 5,107,122 but only for the single detector segment to which this patent relates. Thus, it is clear from FIG. 2 of U.S. Pat. No. 5,107,122 that only a single detector segment is contemplated. Moreover, it is to be noted that the readout chip in U.S. Pat. No. 5,107,122 is configured as an array having a plurality of readout circuits each corresponding to a respective pixel in the sensor chip and being aligned therewith and connected thereto by means of bump contacts. Such an arrangement is particularly convenient for a single detector segment, since neighboring pixels in the sensor chip are directly mapped to neighboring circuits in the readout chip.
However, in practice, it is not always feasible to employ such a structure. For example, in our EP 893 705 there are disclosed multiple detector segments each comprising an array of 16×16 pixels, i.e. 256 pixels per pixel array and each being coupled to a corresponding channel of a one-dimensional ASIC having 256 channels. Each ASIC channel provides pre-amplification, noise-filtering and generation of trigger signals. A trigger is generated whenever the input charge exceeds a certain threshold. If the primary hit alone is detected by the ASIC, the remainder of the charge deposited in other pixels can be recovered by reading out the neighboring pixels. In a one-dimensional detector array, neighboring detector elements are usually connected to neighboring ASIC channels. Reconstruction of physical events in case of charge sharing or Compton scattering can be done by reading out neighboring ASIC channels. However, in a system where pixels are connected to an ASIC having a one-dimensional channel array structure, the two dimensions of the pixel array are mapped into one dimension.
It thus emerges that the simple structure of U.S. Pat. No. 5,107,122 allowing direct mapping of neighboring pixels to neighboring circuit elements in the readout chip is impractical. When mapping a two-dimensional sensor to a one-dimensional ASIC, neighboring pixels, such as those along an edge of the sensor chip that border an active pixel in a different row or column, may map to a channel in the ASIC that is remote from the channel associated with the active pixel. This renders impossible an intuitive understanding as to which channels in the ASIC neighbor the channel associated with the active pixel.
This problem is, of course, exacerbated when multiple detector segments are used. In computer tomography applications, multiple detector segments may be required in order to increase the effective area that can be imaged. Moreover, in Positron Emission Tomography (PET) a patient is administered a radioisotope that emits positrons (i.e. positively charged electrons). When the positrons meet electrons within the patient's body, the positrons and electrons mutually annihilate and produce two γ-rays that propagate away from each other at an angle of 180° and are detected by respective detector segments in the PET scanner. Thus, two detectors are required on opposite sides of the patient. Each of these detectors may, and typically will, comprise multiple sensor segments.
Moreover, when multiple sensor segments are used, potential neighboring pixels may in fact reside in adjacent sensor segments, thus requiring that one or more adjacent sensor segments be sampled. Such sensor segments are typically “dead” for the read out period such that new trigger events are lost. This applies regardless of whether the sensor segments are part of the same detector or belong to different detectors.